Re: CET and MDMT

Samuel,

The +40 DegF you mentioned sounds more like an MDMT for that particular exchanger rather than a CET. Back in that time frame, very few folks used the term CET (Exxon was one of them) and then they often used the terms interchangeably (which is not strictly correct). I've found that even now, folks sometimes have difficulty distinguishing between these two parameters. To give you a couple of reference points, the CET based on ambient conditions (not low temperature process operations) is about +15 DegF in the Houston area and -7 DegF in the Northern New Jersey area where I am.

For your case, you need to find the lowest one-day mean atmospheric temperature for the site where the equipment will go. This will give you the CET based on ambient conditions and will govern in most cases. For equipment that will operate below that minimum ambient temperature, the CET for that particular item will be that lower temperature. For new equipment, you then have to select materials using ASME Code rules such that the MDMT for the item is no higher than the CET that you have determined (or else impact test the material at the CET).

Regards,

Vince

Vincent A. Carucci
Carmagen Engineering, Inc.
4 West Main Street
Rockaway, NJ 07866

Phone: 973-627-4455
FAX: 973-627-3133 Visit our web site at www.carmagen.com
----- Original Message -----
From: "Samuel Liu" <samuel.liu@amec.com> To: <PipingDesign@yahoogroups.com> Sent: Friday, June 29, 2001 3:04 PM
Subject: RE: [PipingDesign] CET and MDMT

> The CET specified as +40 DegF, appeared on one old heat exchanger drawing
> back to 1980, and it may not be used anymore for the current pressure
vessel
> design. The heat exchanger was made in Port Coquitlam, BC. There was no
any
> stipulation for MDMT on all available document for this equipment. What I
> need to know is whether there were/are some relationships between the old
> CET and new MDMT.
>
> Best Regards,
>
> Samuel Liu
>
> -----Original Message-----
> From: Christopher Wright [mailto:chrisw@skypoint.com]
> Sent: Thursday, June 28, 2001 8:32 PM
> To: ?
> Subject: Re: [PipingDesign] CET and MDMT
>
>
> >Does anybody can help me to determine the relationship between CET
> (Critical
> >Exposure Temperature) and MDMT (Minimum Design Metal Temperature)?
> I confess I've never heard the term CET. The MDMT pertains to resistance
> to brittle fracture in carbon and low alloy steel. The MDMT is used to
> determine the need for Charpy impact testing. Just guessing, but I
> daresay the the CET is some sort of upper temperature limit relating to
> creep or metallurgical effect. The MDMT reflects a limit on low
> temperature service.
>
> Christopher Wright P.E. |"They couldn't hit an elephant from
> chrisw@skypoint.com | this distance" (last words of Gen.
> ___________________________| John Sedgwick, Spotsylvania 1864)
> <a href="http://www.skypoint.com/~chrisw">http://www.skypoint.com/~chrisw</a>
>
>
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Received on Mon Jul 02 11:23:00 2001